This invention relates to pressure-sensitive adhesive tapes, especially tapes which contain voids, and more particularly to tapes in which the voids are provided by incorporating hollow microspheroids. The desirability of a tape construction capable of conforming to irregular surfaces has long been recognized, one way of achieving this physical characteristic being illustrated in U.S. Pat. Nos. 3,439,950 and 4,145,465, each of which shows a foamed conformable backing coated on at least one face with a conventional pressure-sensitive adhesive (PSA). U.S. Pat. No. 3,565,247 teaches that the PSA itself may be foamed to impart conformability, U.S. Pat. No. 4,415,615 pointing out that filling the voids in the foamed adhesive with gas reduces permanent compression when the adhesive is subjected to pressure. U.S. Pat. No. 4,223,067 shows that hollow glass microspheres can be incorporated to create a foam-like appearance.
U.S. Pat. No. 3,314,838 shows the incorporation of fragile glass microbubbles at the upper surface of a PSA tape; the bubbles, which are covered with a thin coating of PSA, provide reduced adhesion and permit repositioning until the desired location is achieved, at which time the application of force crushes the bubbles and permits the main body of the adhesive to contact the surface being adhered to. Canadian Pat. No. 1,174,124, German Pat. No. 1,594,060 and U.S. Pat. No. 4,376,151 describe the incorporation of non-adhesive particles at the surface of a PSA to reduce adhesion, the particles migrating into the adhesive when sufficient pressure is applied and thereby increasing the adhesive bond.
Japanese Pat. Pubs. No. 56-61468 and 56-61469 teach the incorporation of expanded or expandable polymeric microbubbles in a pressure-sensitive adhesive. In one construction, the expanded microbubbles cover the surface of the PSA, preventing adhesion until such time as sufficient heat is applied to collapse the bubbles and expose the PSA. In the other construction, expandable microspheres are included in the pressure-sensitive adhesive, the application of heat expanding the microbubbles and destroying the bond of the PSA to a substrate to which it had been applied previously. In each case, it appears that the patentees incorporate a sufficient quantity of microspheres to constitute perhaps 90% of the total volume of the adhesive layer when the spheres are expanded, thereby effectively destroying any adhesive properties. Suitable bubbles for the patentees' purposes are taught in U.S. Pat. Nos. 3,615,972 and 4,287,308. U.S. Pat. No. 4,388,424 discloses caulk or sealant containing glass microballoons, U.S. Pat. Nos. 4,005,033 and 4,075,138 both teaching the incorporation of polymeric microspheres in the same products. Japanese Pat. Pub. No. 56-61467 suggests including unexpanded plastic bubbles in adhesives (including PSAs), interposing the adhesive between two objects to be bonded and applying heat to expand the bubbles and fill gaps between the objects. U.S. Pat. No. 3,864,161 describes the incorporation of 0.25 to 40% by weight unexpanded hollow polymeric microspheres in a solution of a film-forming coating of adhesive polymer, coating the composition on a substrate, drying, and heating to expand the microspheres and form foam-like products (either open or closed cell) that are suggested for decorative coatings, padding, and gaskets.
U.S. Pat. Nos. 3,691,140 and 4,166,152 describe inherently tacky microspheres, which in U.S. Pat. Nos. 4,049,483 and 3,857,731 are used in conjunction with other adhesives, allowing the microspheres to protrude above the surface and thereby provide an adhesion level lower than if the adhesive surface were uniform and smooth; products in which inherently tacky microspheres are bonded to a paper backing are sold by Minnesota Mining and Manufacturing Company as "POST-IT" Brand notes, which can be repeatedly applied to paper, removed, and replaced without delaminating the paper. U.S. Pat. No. 4,588,152 discloses the use of small spaced pressure-sensitive adhesive areas which are said to provide a repositionable tape product.
Alternative approaches to the development of a removable and repositionable pressure-sensitive sheet of this type have included modifying conventional pressure-sensitive adhesives (PSAs) by lowering the initial adhesion and attempting to minimize the subsequent adhesion buildup which occurs as the adhesive remains in contact with the substrate, employing firmer adhesives, minimizing cold flow, and decreasing the total surface area coated with adhesive. Although successful to some extent, each of these techniques is limited in its ability to provide a balance of properties, i.e., sufficient adhesion to ensure attachment while minimizing the ultimate adhesion buildup so as to permit removal. The present invention provides a means of achieving all the advantages of prior art while avoiding the disadvantages. Conventional PSAs can be employed, while avoiding any adhesive transfer, maintaining removability over extended periods, and retaining excellent shear properties.
Another widely used commercial product is so-called "foam tape", in which a foam backing is provided on each face with a coating of PSA. For foam-like tapes, photopolymerizable or thermally polymerizable monomer systems are particularly preferred. These systems preferably comprise one or more monomers that are predominantly alkyl acrylate, the alkyl groups of which have an average of 4 to 12 carbon atoms, and a minor proportion of one or more polar co-polymerizable monomers such as disclosed in U.S. Pat. No. 2,884,126. Tapes of this type are, for example, especially effective in mounting resilient molding to the sides of automobiles to prevent inadvertent damage to the finish. In some cases, the foam backing is replaced with an adhesive that contains glass microbubbles to create a pseudo-foam effect while simultaneously retaining the adhesive properties of the PSA. The latter construction can be modified by laminating an unfoamed PSA to one or both faces. While effective is most circumstances, products of all these types are sensitive to shock at extremely low temperatures.